1. Field of the Invention
This invention relates to (a) new compositions for use in reinforcing thin metal panels and other substrates, (b) new methods for reinforcing such substrates, particularly substrates so structured or positioned as to render known methods unworkable for reinforcing them, and (c) new forms of reinforced substrates.
2. Description of the Prior Art
There are many situations where a need exits for reinforcing a structural member to permit thinner cross sections of material to be used or to provide increased localized strength to a constructed article. For example, in the case of automotive vehicles, various reinforcing methods have been developed and applied to steel panels that form body parts of the vehicle.
A variety of reinforcing methods exist. For example, corrugation or lamination of panels can be used. Also, strengthening members, e.g., struts, can be welded or attached by fasteners to the substrate. However, such methods may not be applicable in many situations, e.g., corrugation of automotive exterior body panels is unacceptable.
Another known class of reinforcement methods involves the attachment of a strengthing member or material by a cement or adhesive to the panel or other substrate. For example, reinforcing material has been adhesively bonded to the inside of body panels such as a roof, a fender, a hood, a trunk, a quarter panel or a door, all of which are relatively broad and have a flat form, but of thin cross section, so that such panels will have sufficient stiffness to withstand stress exerted thereon. Prior reinforcing methods of this type are disclosed in U.S. Pat. Nos. 4,369,608; 4,378,385; 4,444,818 and 4,569,880. The present invention concerns this general type of reinforcement technique which will be referred to herein as adhesive reinforcing systems.
Adhesive reinforcing systems and methods are extensively used in the construction of new vehicles or other items, but are also useable in the repair of damaged items (see U.S. Pat. No. 2,795,523).
Known adhesive reinforcing systems have utilized as components thereof metals, asphalt, rubber, epoxy resins, acrylic resins, phenolic resins, polyester resins and combinations or modifications thereof. Regardless of the components used, requirements of a good adhesive reinforcing system for a given substrate are:
1. Adhesion to the substrate at room temperature with no or minimum surface preparation.
2. Continued adhesion to the substrate under adverse conditions until a heating operation can be reached, if the system requires heating for its completion. For example, panels to be used in automotive applications before a heating operation may need to undergo chemical treatment.
3. Capacity for hardening or curing in a specific temperature and time range.
4. Capability of not slumping, sagging or loosing adhesion in any of these operations.
Prior adhesive reinforcing systems such as those referenced above contain a reinforcement material in an unhardened or semi-hardened sheet that may or may not contain an additional bead forming material or foaming material. Essentially, these systems comprise pasty, reactive resins with some support layer and, when bonded and heated, become an permanent part of the substrate. All such systems have several disadvantages in meeting the listed requirements. For example, pasty reinforcing materials containing a latent curing agent show a very high sensitivity to changing temperature and humidity. Resulting variation in tack affects their adhesion performance. This can be a costly problem in automobile plants where panels with the adhesive reinforcing system on assembly lines lose the adhesive system. Since the temperatures in a variety of assembly plants differ, the success of such adhesive reinforcing systems will be unpredictable. Futher, because of the very hig viscosity of components of such systems, the reinforcement of panels or other substrates that have inaccessible areas, e.g., corners and curvatures, can not be reinforced therewith. This might be solved by the use of systems comprising less viscous resin components, but then a new problem is created, namely, sagging or slumping of the system before hardening. The present invention provides a new adhesive reforcing systems that avoid these problems.
Part of the success of the present invention depends on the use of epoxy resins as a component of the new adhesive reinforcement systems. Epoxy resins have been known for several decades and are an established class of curable resins and they, as well as compositions containing them, are disclosed in numerous patents and publications, e.g., see U.S. Pat. Nos. 2,863,853; 2,890,2041; 2,890,196; 3,001,972 and 3,139,657 and German offenlegungsshrift No. 2923314. The present invention uses a one-part form of epoxy resin, i.e., a resin material to which a catalyst or hardening agent need not be added prior to application, in providing new and improved adhesive reinforcing systems.